Forward extrusion forging apparatus and forward extrusion forging method

ABSTRACT

A forward extrusion forging apparatus includes: a die having a hollow portion whose diameter is reduced at an intermediate position; and a punch that is positioned behind a workpiece placed in the hollow portion, slides in the hollow portion, and extrudes the workpiece ahead of the hollow portion so that the workpiece is forged. An extruding surface of the punch, which extrudes the workpiece, is recessed in a direction opposite an extruding direction in a manner such that a center portion of the extruding surface is most recessed.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2013-235268 filed on Nov. 13, 2013 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a forward extrusion forging apparatus and a forward extrusion forging method.

2. Description of Related Art

In hot forging, there are roughly two processing forms. One of the two processing methods is a method of manufacturing a compact using so-called rearward extrusion (a compact is manufactured while extruding a target material in a direction opposite an extruding direction of a punch), the method including: placing a target material into a die; and extruding a part of the target material into a hollow portion of an extruding punch while pressing the target material with use of the extruding punch to reduce the thickness thereof. The other one of the two processing methods is a method of manufacturing a compact using so-called forward extrusion (a compact is manufactured while extruding a target material in an extruding direction of a punch), the method including: placing a target material into a die having a hollow portion; and extruding a part of the target material from the hollow portion of the die while pressing the target material with use of a punch that does not have a hollow portion, to reduce the thickness thereof.

Rare earth magnets made using rare earth elements such as lanthanoid are called permanent magnets and are used for driving motors of hybrid vehicles, electric vehicles, and the like as well as motors included in hard disks and MRIs.

As an index indicating magnet performance of these rare earth magnets, for example, residual magnetization (residual magnetic flux density) and coercive force may be used. Along with a decrease in the size of a motor and an increase in current density, the amount of heat generation increases, and thus the demand that rare earth magnets to be used should have high heat resistance has further increased. Accordingly, one of the important research issues in this technical field is how to hold magnetic properties of a magnet when being used at a high temperature.

Examples of the rare earth magnets include commonly-used sintered magnets in which a grain size of crystal grains (main phase) constituting a structure thereof is approximately 3 μm to 5 μm; and nanocrystalline magnets in which crystal grains are refined into nano grains with size of approximately 50 nm to 300 nm. Among these, currently, nanocrystalline magnets have attracted attention because they make it possible to reduce the addition amount of expensive heavy rare earth elements (or to eliminate the necessity of adding heavy rare earth elements), and to realize the refinement of crystal grains.

An example of a method of manufacturing a rare earth magnet will be briefly described. For example, a method of manufacturing a rare earth magnet (oriented magnet) is commonly used, this method including: rapidly solidifying Nd—Fe—B molten metal to produce fine powder; press-forming the fine powder into a rare earth magnet precursor (sintered compact); and performing hot working on this sintered compact so as to impart magnetic anisotropy thereto.

In this hot working, the above-described forward extrusion or rearward extrusion may be used. More specifically, in the rearward extrusion, a sintered compact is placed in a die and is extruded and pressed by an extruding punch having a hollow portion, and a rare earth magnet precursor is manufactured while the sintered compact is extruded in a direction opposite an extruding direction. On the other hand, in the forward extrusion, a sintered compact is placed in a die and is pressed by a punch that does not have a hollow portion, and a part of the sintered compact is extruded from the hollow portion of the die, thereby manufacturing a rare earth magnet precursor. No matter which extrusion method is used, in the rare earth magnet precursor which is produced by being pressed by the punch, anisotropy is generated in a direction perpendicular to a pressing direction of the punch.

Thus, there are a plurality of extrusion methods. Hereinafter, a forward extrusion method of related art will be described.

For example, in a forward extrusion forging apparatus which is used in hot working, the diameter of a die having a hollow portion which is included in the apparatus is reduced at an intermediate position thereof, and a sintered compact (workpiece) which is placed in the hollow portion is extruded ahead of the hollow portion so as to be forged by a punch that is positioned behind the workpiece and slides in the hollow portion.

When this apparatus is used and the workpiece is extruded forward by a flat extruding surface of the punch, a shearing friction force is applied between an outer peripheral portion of the workpiece and an inner peripheral portion of the die during extruding. Due to this shearing friction force, it is difficult to smoothly extrude the workpiece forward. In this regard, such an external force is not applied to the center portion of the workpiece and thus the center portion of the workpiece tends to be extruded smoothly as compared to the outer peripheral portion. As a result, the center portion of the sintered compact is extruded before the outer peripheral portion thereof is extruded. Due to a difference in extruding easiness between the outer peripheral portion and the center portion, non-uniform strain distribution and orientation disorder occur in the compact. Therefore, there is a problem in that it is difficult to manufacture a rare earth magnet having high anisotropy.

Japanese Patent Application Publication No. 2000-343171 (JP 2000-343171 A) discloses a forging method using a forging apparatus in which a distal end surface of a punch, which slides in a die whose diameter is reduced stepwise in an extruding direction, has an inclined edge which is obliquely recessed from the periphery thereof. According to this forging method, the occurrence of burrs which may be formed at an end portion of a forged product can be reduced by pressing a peripheral edge of a target material toward the center of the punch with a peripheral edge of the punch.

In the punch included in the forging apparatus disclosed in JP 2000-343171 A, the peripheral edge is recessed in a ring shape, and the center has a flat extruding surface which protrudes forward. Accordingly, even if this forging apparatus is used, it is difficult to solve the problem of the existing forward extrusion method of related art, that is, the problem that it is difficult to manufacture a rare earth magnet having high anisotropy, because the center portion of the sintered compact is extruded by a punch before the outer peripheral portion thereof is extruded, and as a result, orientation disorder occurs in the compact due to a difference in extruding easiness between the outer peripheral portion and the center portion.

SUMMARY OF THE INVENTION

The invention provides a forward extrusion forging apparatus and a forward extrusion forging method in which a center portion of a target material which is placed in a die and an outer peripheral portion of the target material which is likely to receive a shearing friction force from the die can be extruded as uniformly as possible, and thus, a compact where non-uniform strain distribution is not likely to occur can be manufactured.

A first aspect of the invention relates to a forward extrusion forging apparatus including: a die having a hollow portion whose diameter is reduced at an intermediate position; and a punch that is positioned behind a workpiece placed in the hollow portion, slides in the hollow portion, and extrudes the workpiece ahead of the hollow portion so that the workpiece is forged, wherein an extruding surface of the punch, which extrudes the workpiece, is recessed in a direction opposite an extruding direction in a manner such that a center portion of the extruding surface is most recessed.

The forward extrusion forging apparatus according to the first aspect of invention has a distinctive characteristic in the extruding surface of the punch, which extrudes a workpiece, and includes the punch whose extruding surface is recessed in the direction opposite the extruding direction in a manner such that the center portion of the extruding surface is most recessed. By using the punch having such a structure, an outer peripheral portion of the workpiece (portion to which a shearing friction force is applied from an inner peripheral surface of the die) can be extruded before a center portion of the workpiece is extruded. As a result, the extrusion amounts of the center portion and the outer peripheral portion of the workpiece can be made as uniform as possible. By making the extrusion amounts of the center portion and the outer peripheral portion of the workpiece uniform, non-uniform strain distribution in a manufactured forged product can be suppressed, and the high-quality forged product can be manufactured. When this workpiece is a rare earth magnet precursor, a forged product to be manufactured is an oriented magnet. Thus, an oriented magnet having a high orientation degree and high anisotropy can be manufactured, since non-uniform strain distribution is not likely to occur.

In addition, strains are uniformly introduced into the entire forged product manufactured as described above. Therefore, the entire forged product can be effectively used, and the product yield can be improved. For example, when a forged product is an oriented magnet, in forward extrusion of related art, strains are not sufficiently introduced into an outer peripheral portion of the oriented magnet, which comes into contact With a die. Accordingly, the outer peripheral portion having a low orientation degree may not be used as a product. By applying the forward extrusion forging apparatus according to the above-described aspect of the invention to this problem, strains can be uniformly introduced into an inner portion of the entire oriented magnet that is manufactured. Therefore, the entire or substantially entire oriented magnet that is manufactured can be used as a magnet product.

Here, with regard to the structure “the extruding surface of the punch, which extrudes the workpiece, is recessed in the direction opposite the extruding direction in a manner such that the center portion of the extruding surface is most recessed”, specific examples of, the structure include a semi-circular shape having a predetermined radius of curvature, an arc shape having a predetermined central angle, a shape obtained by combining a plurality of arcs having different radii of curvature, a semi-elliptical shape which is long in a radial direction of the extruding surface of the punch, and a semi-elliptical shape which is long in a thickness direction (extruding direction) of the punch.

The punch may be extruded using an actuator such as a motor or a cylinder mechanism or may be manually extruded.

The forward extrusion forging apparatus according to the above-described aspect of the invention can be produced by the simple structure change in which only the extruding surface of the punch is improved. Therefore, it is possible to suppress an increase in the cost of producing the apparatus.

A second aspect of the invention relates to a forward extrusion forging method including: preparing a forward extrusion forging apparatus in which a die has a hollow portion, a diameter of the hollow portion is reduced at an intermediate position of the hollow portion, a workpiece placed in the hollow portion is extruded ahead of the hollow portion so as to be forged by a punch that is positioned behind the workpiece and slides in the hollow portion, and an extruding surface of the punch, which extrudes the workpiece, is recessed in a direction opposite an extruding direction in a manner such that a center portion of the extruding surface is most recessed; and producing a first forged product by placing a first workpiece in the hollow portion and extruding the first workpiece forward with use of the punch so that the first workpiece is forged.

In the forward extrusion forging method according to the second aspect of the invention, the above-described forward extrusion forging apparatus is used, and extrusion forging is performed using the forward extrusion forging apparatus including the punch whose extruding surface is recessed in the direction opposite the extruding direction in, a manner such that the center portion of the extruding surface is most recessed. Accordingly, the outer peripheral portion of the workpiece can be extruded before the center portion thereof is extruded, and the extrusion amounts of the center portion and the outer peripheral portion of the workpiece can be made as uniform as possible. As a result, occurrence of non-uniform strain distribution can be suppressed in a forged product, and the high-quality forged product can be manufactured.

Before the first workpiece is placed in the hollow portion, a forward surface of the first workpiece in the extruding direction may be recessed in the direction opposite the extruding direction in a manner such that a center portion of the forward surface of the first workpiece is most recessed.

By using the workpiece whose forward surface in the extruding direction is recessed in the direction opposite the extruding direction in a manner such that the center portion of the forward surface is most recessed, in addition to the punch whose extruding surface is recessed in the direction opposite the extruding direction in a manner such that the center portion of the extruding surface is most recessed, the outer peripheral portion of the workpiece can be more effectively extruded forward before the center portion thereof is extruded.

Before the first workpiece is placed in the hollow portion, a rearward surface of the first workpiece, which is pressed by the punch, may protrude in the direction opposite the extruding direction in a manner such that a center portion of the rearward surface of the first workpiece is most protruding.

Since the rearward surface of the workpiece protrudes in the direction opposite the extruding direction in a manner such that the center portion of the rearward surface is most protruding, the rearward surface of the workpiece is fitted to the extruding surface of the punch, and thus the workpiece can be easily extruded by the punch. From this point of view, it is more preferable that the shape of the rearward surface of the workpiece and the shape of the extruding surface of the punch be complementary to each other.

The forward extrusion forging method according to the above-described aspect may further include: producing, after the first forged product is produced, a second forged product by placing a second workpiece in the hollow portion behind the first forged product and extruding the second workpiece forward with use of the punch so that the second workpiece is forged, and extruding the first forged product from an exit of the hollow portion with use of the second forged product.

The forging method according to the above-described aspect is a so-called consecutive extrusion forging method in which a plurality of workpieces are consecutively added by extruding a workpiece and continuously extruding another workpiece.

During this consecutive extrusion forging, the first and second workpieces are consecutively extruded by the above-described punch whose extruding surface, which extrudes the workpiece, is recessed in the direction opposite the extruding direction in a manner such that the center portion of the extruding surface is most recessed. As a result, it is possible to reduce a situation where a workpiece is added while being embedded into another workpiece in front of the workpiece in an arrowhead manner.

In a method of related art in which a workpiece is extruded so as to be forged by a punch whose extruding surface is flat, a center portion of the workpiece is likely to be extruded first as described above. Therefore, in consecutive extrusion forging, a center portion of a workpiece is likely to be embedded into another workpiece in front of the workpiece in the arrowhead manner. It has been confirmed that, when a workpiece is embedded into another workpiece in front of the workpiece in the arrowhead manner, an arrowhead-shaped portion that is formed is likely to be cracked. Accordingly, this arrowhead-shaped portion causes a decrease in product yield.

Regarding this problem, the second workpiece behind the first workpiece is extruded using the punch whose extruding surface extruding the workpiece is recessed in the direction opposite the extruding direction in a manner such that the center portion of the extruding surface is most recessed. As a result, the center portion and the outer peripheral portion of the second workpiece can be extruded as uniformly as possible, and the formation of the arrowhead-shaped portion can be effectively reduced.

Before the second workpiece is placed in the hollow portion, a forward surface of the second workpiece in the extruding direction may be recessed in the direction opposite the extruding direction in a manner such that a center portion of the forward surface of the second workpiece is most recessed.

Before the second workpiece is placed in the hollow portion, a rearward surface of the second workpiece, which is pressed by the punch, may protrude in the direction opposite the extruding direction in a manner such that a center portion of the rearward surface of the second workpiece is most protruding.

The workpiece may be a sintered compact produced by press-forming powder that is a rare earth magnet material, the powder may include a RE-Fe—B-based main phase and a grain boundary phase of a RE-X alloy present around the main phase, RE may represent at least one of Nd and Pr, X may represent a metal element, and a rare earth magnet may be manufactured by performing hot working in which anisotropy is imparted to the sintered compact by forward extrusion forging.

Examples of the rare earth magnet to be manufactured by the forging method according to the above-described aspect of the invention include a nanocrystalline magnet in which a grain size of a main phase (crystal) constituting a structure thereof is approximately 200 nm or less; a magnet having a grain size of 300 nm or more; a sintered magnet having a grain size of 1 μm or more; and a bonded magnet in which crystal grains are bonded through a resin binder. It is preferable that the size of the main phase of magnetic powder before hot working be adjusted such that an average maximum size (average maximum grain size) of the main phase of a rare earth magnet which is manufactured as a final product is approximately 300 nm to 400 nm or less.

An example of a method of manufacturing a rare earth magnet will be described. A quenched thin band (quenched ribbon) which includes fine crystal grains is prepared by liquid quenching. Magnetic powder for a rare earth magnet is prepared by, for example, crushing the quenched ribbon. This magnetic powder is filled into, for example, a die and is sintered while being pressed by a punch so as to be bulked. As a result, an isotropic sintered compact is produced.

For example, this sintered compact has a metallographic structure that includes a RE-Fe—B-based main phase (RE: at least one of Nd and Pr, more specifically, one element or two or more elements selected from among Nd, Pr, and Nd—Pr) with a nanocrystalline structure and a grain boundary phase of an RE-X alloy (X: metal element) present around the main phase.

A rare earth magnet which is an oriented magnet is manufactured by performing hot extrusion forging (hot working) in which anisotropy is imparted to the sintered compact that is manufactured.

As can be seen from the above description, with the forward extrusion forging apparatus and the forward extrusion forging method according to the above-described aspects of the invention, the outer peripheral portion of the workpiece (portion to which a shearing friction force is applied from the inner peripheral surface of the die) can be extruded before the center portion thereof is extruded. As a result, the extrusion amounts of the center portion and the outer peripheral portion of the workpiece can be made as uniform as possible. Thus, occurrence of non-uniform strain distribution in a manufactured forged product can be suppressed, and the high-quality forged product can be manufactured. In addition, when the workpiece is a sintered compact produced by press-forming powder which is a rare earth magnet material, and a rare earth magnet is manufactured by performing hot working in which anisotropy is imparted to the sintered compact by forward extrusion forging, a rare earth magnet having a high orientation degree and high residual magnetization can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a longitudinal sectional view illustrating a forward extrusion forging apparatus according to the invention;

FIGS. 2A to 2C are longitudinal sectional views each illustrating a punch included in the forward extrusion forging apparatus;

FIGS. 3A and 3B are flow diagrams illustrating a forward extrusion forging method according to Embodiment 1 of the invention in order of FIG. 3A and FIG. 3B;

FIG. 4 is a diagram illustrating a state where a forged product is manufactured using the forging method illustrated in FIGS. 3A and 3B;

FIG. 5 is a diagram illustrating a forging method in which a forward extrusion forging apparatus of related art is used.

FIGS. 6A and 6B are diagrams each illustrating a forward extrusion forging apparatus and a workpiece according to the other embodiment that is placed in a die;

FIGS. 7A and 7B are flow diagrams illustrating the forward extrusion forging method according to Embodiment 2 of the invention in order of FIG. 7A and FIG. 7B;

FIG. 8 is a diagram illustrating a consecutive extrusion forging method in which a forward extrusion forging apparatus of related art is used;

FIGS. 9A and 9B are flow diagrams illustrating a process of preparing quenched thin bands (quenched ribbons), a process of preparing magnetic powder for a rare earth magnet by, for example, crushing the quenched ribbons, and a process of pressing the magnetic powder to produce an isotropic sintered compact in order of FIG. 9A and FIG. 9B;

FIG. 10 is a diagram illustrating a microstructure of a sintered compact which is manufactured according to the manufacturing flow in FIGS. 9A and 9B;

FIG. 11 is a diagram illustrating a microstructure of a rare earth magnet which is manufactured by performing forward extrusion forging on the sintered compact illustrated in FIG. 10;

FIG. 12 is a diagram illustrating the results of experiment of measuring processing strain at each site of a compact which was manufactured using a forward extrusion forging apparatus of related art and compacts which were manufactured using a forward extrusion forging apparatus according to the invention; and

FIG. 13 is a diagram illustrating the results of experiment of measuring the lengths of arrowhead-shaped portions of a compact which was manufactured using the forward extrusion forging apparatus of related art and a compact which was manufactured using the forward extrusion forging apparatus according to the invention, the arrowhead-shaped portions being formed during consecutive extrusion forging.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of a forward extrusion forging apparatus and a forward extrusion forging method according to the invention will be described with reference to the drawings.

(Forward Extrusion Forging Apparatus) FIG. 1 is a longitudinal sectional view illustrating a forward extrusion forging apparatus according to the invention. A forging apparatus 10 illustrated in FIG. 1 includes a die 1 having a hollow portion H, a punch 2 that is slidable in the hollow portion H of the die 1, and a heating device (not illustrated) that heats an actuator (not illustrated) that slides the punch 2 and the die 1 during forging.

A diameter of the hollow portion H of the die 1 is reduced at an intermediate position thereof. A workpiece (not illustrated) placed in the hollow portion H is extruded ahead of the hollow portion H (in an X direction) by the punch 2 which is positioned behind the workpiece and slides along an inner peripheral surface 1′ of the die 1 in the hollow portion H. The workpiece is extruded from an extruding port 1″ so as to be forged.

The diameter of the hollow portion H of the die 1 is reduced at the intermediate position thereof, and the workpiece is extruded through this diameter-reduced portion so as to be forged. In the example illustrated in the drawing, the die has one diameter-reduced portion but may have two or more diameter-reduced portions with different diameters.

In the punch 2, an extruding surface 2′ that extrudes the workpiece is recessed (concaved) in a direction opposite an extruding direction (X direction) in a manner such that a center portion of the extruding surface 2′ is most recessed. The shape of the extruding surface 2′ of the punch 2 illustrated in FIG. 1 is an arc shape having a radius of curvature R1. An extruding surface of a punch included in an apparatus of related art is flat. In contrast, the extruding surface 2′ of the punch 2 included in the forging apparatus 10 according to the invention has a novel structure, that is, the extruding surface 2′ is recessed in the direction opposite the extruding direction in a manner such that the center portion of the extruding surface 2′ is most recessed as illustrated in the drawing, and this structure is not present in the apparatus of related art.

FIGS. 2A to 2C illustrate other embodiments regarding the shape of the extruding surface.

An extruding surface 2 a of a punch 2A illustrated in FIG. 2A has a shape obtained by combining two kinds of arcs that have radii of curvature of R2 and R3, respectively.

On the other hand, an extruding surface 2 b of a punch 2B illustrated in FIG. 2B has a semi-elliptical shape which is long in a radial direction of the punch 2B.

On the other hand, an extruding surface 2 c of a punch 2C illustrated in FIG. 2C has a semi-elliptical shape which is long in a thickness direction of the punch 2C (extruding direction: X direction).

When any punch among the punches with the extruding surfaces according to the above-described embodiments is used, an outer peripheral portion (a portion to which a shearing friction force is applied from the inner peripheral portion 1′ of the die 1) of the workpiece can be extruded before the center portion of the workpiece is extruded, since the extruding surface, which extrudes the workpiece, is recessed in the direction opposite the extruding direction (X direction) in a manner such that the center portion of the extruding surface is most recessed. Accordingly, the outer peripheral portion of the workpiece, which is difficult to smoothly extrude due to the shearing friction force, can be extruded before the center portion is extruded. As a result, the extrusion amounts of the center portion and the outer peripheral portion of the workpiece can be made as uniform as possible. Thus, occurrence of non-uniform strain distribution in a manufactured forged product can be suppressed, and the high-quality forged product can be manufactured.

(Forward Extrusion Forging Method in Embodiment 1) FIGS. 3A and 3B are flow diagrams illustrating a forward extrusion forging method according to Embodiment 1 of the invention in order of FIG. 3A and FIG. 3B.

First, as illustrated in FIG. 3A, the forging apparatus 10 illustrated in FIG. 1 is prepared, and a workpiece W is placed in the hollow portion H of the die 1 of the forging apparatus 10 (first step).

Next, as illustrated in FIG. 3B, in a state where the inside of the hollow portion H is heated to a predetermined temperature, the punch 2 is caused to slide in the hollow portion H, and the workpiece W is extruded forward so as to be forged by the extruding surface 2′ of the punch 2. As a result, a forged product WT is manufactured as illustrated in FIG. 4 (second step).

In the process of extrusion by the punch 2, the outer peripheral portion of the workpiece W receives a shearing friction force F from the inner peripheral portion 1′ of the die 1 and is difficult to smoothly extrude as compared to the center portion of the workpiece W.

Here, FIG. 5 illustrates a state where a workpiece is extruded so as to be forged using a forging apparatus of related art.

In the forging apparatus of related art, a punch Pa having a flat extruding surface is caused to slide in a die Di and extrudes the workpiece W so as to be forged. As in the case where the forging apparatus 10 according to the invention illustrated in FIG. 3B is used, a shearing friction force F is applied to the outer peripheral portion of the workpiece W and the outer peripheral portion of the workpiece W is not smoothly extruded as compared to the center portion of the workpiece W. When the workpiece W is extruded by the punch Pa having the flat extruding surface in this state, the center portion of the workpiece W, which is likely to be smoothly extruded, is extruded before the outer peripheral portion is extruded. Thus, as illustrated in FIG. 5, the workpiece W is extruded so as to be forged in a state in which the center portion protrudes in the extruding direction (X direction). As a result, non-uniform strain distribution is likely to occur in a manufactured forged product. This affects the quality of the forged product.

In contrast, with the forging apparatus 10 according to the invention, the outer peripheral portion (the portion to which a shearing friction force F is applied from the inner peripheral portion 1′ of the die 1) of the workpiece W can be extruded before the center portion of the workpiece W is extruded, since the extruding surface 2′ of the punch 2, which extrudes the workpiece W, is recessed in the direction opposite the extruding direction (X direction) in a manner such that the center portion of the extruding surface 2′ is most recessed. Accordingly, the outer peripheral portion of the workpiece which is difficult to be smoothly extruded due to the shearing friction force F can be extruded before the center portion is extruded. As a result, the extrusion amounts of the center portion and the outer peripheral portion of the workpiece can be made as uniform as possible. Thus, occurrence of non-uniform strain distribution in a manufactured forged product can be suppressed, and the high-quality forged product can be manufactured.

(Workpiece to Be Extruded and Forged in Other Embodiments) FIGS. 6A and 6B are diagrams each illustrating a forward extrusion forging apparatus and a workpiece according to the other embodiment that is placed in the die of the forging apparatus.

In a workpiece Wa illustrated in FIG. 6A, a forward surface in an extruding direction (X direction) is recessed in a direction opposite the extruding direction in a manner such that a center portion of the forward surface is most recessed. As in the case of the plurality of shapes of the extruding surfaces of the punches illustrated in FIGS. 2A, 2B, and 2C, various shapes can be employed as the shape of the forward surface of the workpiece, as long as the forward surface of the workpiece is recessed in the direction opposite the extruding direction in a manner such that the center portion of the forward surface is most recessed.

By using not only the extruding surface 2′ of the punch 2 included in the forging apparatus 10 but also the workpiece Wa illustrated in FIG. 6A, an outer peripheral portion of the workpiece Wa can be more effectively extruded forward before a center portion thereof is extruded, and the extrusion amounts of the center portion and the outer peripheral portion of the workpiece Wa can be made uniform.

On the other hand, in a workpiece Wb illustrated in FIG. 6B, as in the case of the workpiece Wa, a forward surface in an extruding direction (X direction) is recessed in a direction opposite the extruding direction in a manner such that a center portion of the forward surface is most recessed. Further, a rearward surface of the workpiece Wb, which is pressed by the punch 2, protrudes (in other words, the rear surface is convex) in the direction opposite the extruding direction in a manner such that a center portion of the rearward surface is most protruding. The shape of the rearward surface of the workpiece Wb is complementary to the shape of the extruding surface 2′ of the punch 2. When the rearward surface of the workpiece Wb is fitted into the extruding surface 2′ of the punch 2, both the surfaces come into contact with each other, and the workpiece Wb is easily extruded by the punch 2.

Thus, by improving the shape of a forward surface and/or the shape of a rearward surface of a workpiece to be extruded and forged in an extruding direction as in the examples illustrated in the drawings, the extrusion amounts of the portions of the workpiece can be made as uniform as possible, due to the combined effect of the improved shape of the forward surface of the workpiece and/or the improved shape of the rearward surface of the workpiece and the shape of the extruding surface of the punch. Accordingly, a high-quality forged product having no non-uniform strain distribution or slightly non-uniform strain distribution can be manufactured.

(Forward Extrusion Forging Method in Embodiment 2) FIGS. 7A and 7B are flow diagrams illustrating the forward extrusion forging method according to Embodiment 2 of the invention in order of FIG. 7A and FIG. 7B, that is, illustrating a relay extrusion forging method in which plural forged products are manufactured while being successively added.

First, as illustrated in FIG. 7A, the forging apparatus 10 illustrated in FIG. 1 is prepared, a workpiece W is placed in the hollow portion H of the die 1 of the forging apparatus 10, and the punch 2 is caused to slide. As a result, a first forged product WT1 is manufactured (first and second steps). Next, the punch 2 is returned rearward, and another workpiece W is placed in the hollow portion H.

Next, the workpiece W is extruded forward so as to be forged by causing the punch 2 to slide again, and a second forged product WT2 is consecutively added behind the first forged product WT1 (third step).

Here, an arrowhead-shaped portion Ya is generally formed at an tip end of the second forged product WT2, and this arrowhead-shaped portion Ya is embedded into the first forged product WT1.

This arrowhead-shaped portion Ya has a weaker strength than the other portions of the forged product and thus is likely to be cracked. Therefore, a range (range of a width t1 in FIG. 7B) where the arrowhead-shaped portion Ya is present cannot be used as a final product and causes a decrease in product yield. Accordingly, it is preferable that consecutive extrusion forging be performed such that the arrowhead-shaped portion Ya illustrated in the drawing is as small as possible.

FIG. 8 is a diagram illustrating a consecutive extrusion forging method in which a forward extrusion forging apparatus of related art is used.

As illustrated in the drawing, since the extruding surface of the punch Pa included in the forging apparatus is flat, the center portion of the workpiece is extruded first. As a result, the second forged product WT2 in which the length t2 (t2>t1) of an arrowhead-shaped portion Ya′ protruding forward is long is manufactured behind the first forged product WT1 which has been previously produced.

The length t1 of the arrowhead-shaped portion Ya illustrated in FIG. 7B is significantly shorter than the length t2 of the arrowhead-shaped portion Ya′ illustrated in FIG. 8, and thus the product yield can be significantly improved.

(Method of Manufacturing Rare Earth Magnet) Next, a method of manufacturing a rare earth magnet will be briefly described with reference to FIGS. 9A to 11. FIGS. 9A and 9B are flow diagrams illustrating a process of preparing quenched thin bands (quenched ribbons), a process of preparing magnetic powder for a rare earth magnet by, for example, crushing the quenched ribbons, and a process of pressing the magnetic powder to produce an isotropic sintered compact in order of FIG. 9A and FIG. 9B. In addition, FIG. 10 is a diagram illustrating a microstructure of a sintered compact which is manufactured according to the manufacturing flow in FIGS. 9A and 9B. In addition, FIG. 11 is a diagram illustrating a microstructure of a rare earth magnet which is manufactured by performing forward extrusion forging on the sintered compact illustrated in FIG. 10.

As illustrated in FIG. 9A, in a furnace (not illustrated) with an Ar gas atmosphere in which the pressure is reduced to, for example, 50 kPa or less, an alloy ingot is melted by high-frequency heating using a single-roll melt spinning method, and molten metal having a composition of a rare earth magnet is injected to a copper roll R to prepare quenched ribbons B, and this quenched ribbons B are crushed.

Among particles of the crushed quenched ribbons 13, particles having a maximum particle size of about 200 nm or less are selected. As illustrated in FIG. 9B, these particles are filled into a cavity which is defined by a cemented carbide die D and a cemented carbide punch P sliding in the hollow portion. Next, the particles are heated by causing a current to flow therethrough in a pressing direction (Z direction) while being pressed with use of the cemented carbide punch P. As a result, a quadrangular prism-shaped sintered compact S is prepared, the sintered compact including: an Nd—Fe—B main phase (having a grain size of about 50 nm to 200 nm) with a nanocrystalline structure; and a grain boundary phase of a Nd—X alloy (X: metal element) present around the main phase.

Here, the Nd—X alloy constituting the grain boundary phase is an alloy of Nd and at least one of Co, Fe, Ga, and the like and is in a Nd-rich state. For example, one alloy or a mixture of two or more alloys selected from among Nd—Co, Nd—Fe, Nd—Ga, Nd—Co—Fe, and Nd—Co—Fe—Ga may be used.

As illustrated in FIG. 10, the sintered compact S has an isotropic crystal structure in which the grain boundary phase BP is filled between nanocrystalline particles MP (main phase).

Once the quadrangular prism-shaped sintered compact S is prepared, hot working is performed thereon by forward extrusion illustrated in FIGS. 3A and 3B to impart magnetic anisotropy to the sintered compact S. As a result, a rare earth magnet (oriented magnet) is manufactured.

For example, when the thickness of a sintered compact before hot working is represented by s1 and the thickness of a rare earth magnet manufactured by hot working is represented by s2, a processing rate is represented by (s1−s2)/s1. It is preferable that hot working be performed by extrusion forging at a processing rate of 60% to 80%. It is preferable that a strain rate during the extrusion of hot working be adjusted to be 0.1/sec or higher.

As illustrated in FIG. 11, by performing hot working by extrusion forging, in a manufactured rare earth magnet C (oriented magnet), the nanocrystalline particles MP have a flat shape, the boundary surface which is substantially parallel to an anisotropic axis is curved or bent. This rare earth magnet C has high magnetic anisotropy.

The oriented magnet C illustrated in the drawing has a metallographic structure that includes a RE-Fe—B-based main phase (RE: at least one of Nd and Pr) and a grain boundary phase of an RE-X alloy (X: metal element) present around the main phase. In the oriented magnet C, a content ratio of RE is 29 mass %≦RE≦32 mass %, and an average grain size of the main phase of the manufactured rare earth magnet is preferably 300 nm. Since the content ratio of RE is in the above-described range, an effect of suppressing cracking during hot working can be further increased, and a high orientation degree can be ensured. In addition, since the content ratio of RE is in the above-described range, it is possible to ensure the size of the main phase at which high residual magnetic flux density can be ensured.

In addition, the coercive force of the manufactured rare earth magnet (oriented magnet) may be further increased by grain boundary diffusion of a heavy rare earth metal such as Dy, and a modified alloy that does not contain a heavy rare earth metal such as a Nd—Cu alloy, a Nd—Al alloy, a Pr—Cu alloy, or a Pr—Al alloy, in the rare earth magnet. For example, the eutectic temperature of the Nd—Cu alloy is approximately 520° C., the eutectic temperature of the Pr—Cu alloy is approximately 480° C., the eutectic temperature of the Nd—Al alloy is approximately 640° C., and the eutectic temperature of the Pr—Al alloy is approximately 650° C. Since the eutectic temperatures of the above modified alloys are significantly lower than a range of 700° C. to 1000° C. in which crystal grains constituting the nanocrystalline magnet are coarsened, the modified alloys are particularly preferably used when the rare earth magnet is a nanocrystalline magnet.

Description will be provided on experiment and results of measuring processing strain at each site of a compact which was manufactured using the forward extrusion forging apparatus of related art and compacts which were manufactured using the forward extrusion forging apparatus according to the invention. The present inventors conducted an experiment of measuring processing strain at each site of a compact (Comparative Example 1) which was manufactured using the forward extrusion forging apparatus of related art and compacts (Examples 1 and 2) which were manufactured using the forward extrusion forging apparatus according to the invention.

In each of Comparative Example 1 and Examples 1 and 2, two workpieces were subjected to consecutive extrusion forging. The sectional shapes of the workpieces of Comparative Example 1 and Example 1 were rectangular, and the sectional shapes of a forward surface and a rearward surface of the workpiece of Example 2 were curved as illustrated in FIG. 6B. The results are shown in FIG. 12.

In FIG. 12, a measurement position of 0 mm is the position of an inner peripheral surface at a diameter-reduced portion of a die, and a measurement position of 1.8 mm is the center position at the diameter-reduced portion of the die.

It was found from FIG. 12 that the processing strain distribution of Example 1 in the entire workpiece was more uniform than that of Comparative Example 1.

In addition, it was found that the processing strain distribution of Example 2 in the entire workpiece was more uniform than that of Example 1.

It can be confirmed from the above experiment results that non-uniform strain distribution in a forged product can be alleviated by performing forward extrusion forging using the forging apparatus according to the present invention which has a distinctive characteristic in the shape of the extruding surface of the punch. Further, it can be confirmed that non-uniform strain distribution in a forged product can be further alleviated by adding an improvement to the shape of a forward surface and a rearward surface of a workpiece.

Description will be provided on experiment and results of measuring the lengths of arrowhead-shaped portions of a compact which was manufactured using the forward extrusion forging apparatus of related art and a compact which was manufactured using the forward extrusion forging apparatus according to the invention, the arrowhead-shaped portions being formed during the consecutive extrusion forging. The present inventors conducted an experiment of measuring and comparing the lengths of arrowhead-shaped portions of a compact (Comparative Example 2) which was manufactured using the forward extrusion forging apparatus of related art and a compact (Example 3) which was manufactured using the forward extrusion forging apparatus according to the invention, the arrowhead-shaped portions being formed during the consecutive extrusion forging. In this experiment, three lubricants having friction coefficients of 0.1, 0.08, and 0.06 were applied to the inner peripheral surfaces of the dies. The results are shown in FIG. 13.

It can be confirmed from FIG. 13 that, at all the friction coefficients, the length of the arrowhead-shaped portion of Example 3 was significantly reduced as compared to that of Comparative Example 2, specifically, the length of the arrowhead-shaped portion of Example 3 was reduced by approximately 50% at the friction coefficients of 0.1 and 0.08 and was reduced by approximately 90% at the friction coefficient of 0.06 as compared to that of Comparative Example 2.

Hereinabove, the embodiments of the invention have been described with reference to the drawings. However, a specific configuration is not limited to the embodiments, and design changes and the like which are made within a range not departing from the scope of the invention are included in the invention. 

What is claimed is:
 1. A forward extrusion forging apparatus comprising: a die having a hollow portion whose diameter is reduced at an intermediate position; and a punch that is positioned behind a workpiece placed in the hollow portion, slides in the hollow portion, and extrudes the workpiece ahead of the hollow portion so that the workpiece is forged, wherein an extruding surface of the punch, which extrudes the workpiece, is recessed in a direction opposite an extruding direction in a manner such that a center portion of the extruding surface is most recessed.
 2. A forward extrusion forging method comprising: preparing a forward extrusion forging apparatus in which a die has a hollow portion, a diameter of the hollow portion is reduced at an intermediate position of the hollow portion, a workpiece placed in the hollow portion is extruded ahead of the hollow portion so as to be forged by a punch that is positioned behind the workpiece and slides in the hollow portion, and an extruding surface of the punch, which extrudes the workpiece, is recessed in a direction opposite an extruding direction in a manner such that a center portion of the extruding surface is most recessed; and producing a first forged product by placing a first workpiece in the hollow portion and extruding the first workpiece forward with use of the punch so that the first workpiece is forged.
 3. The forward extrusion forging method according to claim 2, wherein before the first workpiece is placed in the hollow portion, a forward surface of the first workpiece in the extruding direction is recessed in the direction opposite the extruding direction in a manner such that a center portion of the forward surface of the first workpiece is most recessed.
 4. The forward extrusion forging method according to claim 3, wherein before the first workpiece is placed in the hollow portion, a rearward surface of the first workpiece, which is pressed by the punch, protrudes in the direction opposite the extruding direction in a manner such that a center portion of the rearward surface of the first workpiece is most protruding.
 5. The forward extrusion forging method according to claim 2, further comprising: producing, after the first forged product is produced, a second forged product by placing a second workpiece in the hollow portion behind the first forged product and extruding the second workpiece forward with use of the punch so that the second workpiece is forged, and extruding the first forged product from an exit of the hollow portion with use of the second forged product.
 6. The forward extrusion forging method according to claim 5, wherein before the second workpiece is placed in the hollow portion, a forward surface of the second workpiece in the extruding direction is recessed in the direction opposite the extruding direction in a manner such that a center portion of the forward surface of the second workpiece is most recessed.
 7. The forward extrusion forging method according to claim 6, wherein before the second workpiece is placed in the hollow portion, a rearward surface of the second workpiece, which is pressed by the punch, protrudes in the direction opposite the extruding direction in a manner such that a center portion of the rearward surface of the second workpiece is most protruding.
 8. The forward extrusion forging method according to claim 2, wherein the workpiece is a sintered compact produced by press-forming powder that is a rare earth magnet material, the powder includes a RE-Fe—B-based main phase and a grain boundary phase of a RE-X alloy present around the main phase, RE represents at least one of Nd and Pr, X represents a metal element, and a rare earth magnet is manufactured by performing hot working in which anisotropy is imparted to the sintered compact by forward extrusion forging. 